Jeremy W Bowman

No male can be certain that he really is the father of the offspring he treats as his own. In some species paternity is very uncertain, but the degree of paternal certainty or uncertainty doesn't create any discrepancy between the respective interest that male and female have in marriage-type arrangements. For any given species, the possible gains, losses and risks run by male and female are equal, at least as far as reproductive success is concerned.

Suppose a given species exhibits a particular amount of male parental investment. That amount is a pretty good measure of how much marriage matters to both sexes. First, let's consider the possible gains and losses.

If a species exhibits a particular amount of male parental investment, its offspring's chances of survival are boosted by a particular factor. This quantity differs greatly from one species to the next, but if it is significantly greater than zero, the female gains something in reproductive success from marriage-type arrangements. But so too does the male, because his offspring also gain from those arrangements, whether or not these offspring of his are part of his own family or are provisioned by another, cuckolded male. And the male parent gains the same amount as the female parent, because he is as closely related to his own offspring as she is to her offspring.

Of course, a male who is himself cuckolded loses some reproductive success through expending energy on another male's offspring. It may be tempting to think that because this cannot happen to females, the latter must enjoy a special advantage in the "marriage game".

But that is not so. The more cheating there is, the more suspicion there is, and the more such suspicions are acted on. A male acts on his suspicions by failing to provide for his wife's offspring, or by abandoning her altogether (In species where cheating is rife, the latter is normally easy to arrange.)

So in marriage, the male runs the risk of losing time and energy that he could have spent providing for his own offspring, whereas the female runs the risk of losing provisioning for her actual offspring. Over the long run, these possible losses are equal. The biological cost to the male of female cheating is the same as the biological cost to the female of male abandonment.

Next, consider how much of a risk each sex takes when they enter into marriage-type arrangements. The risk for the male is that he might provide for offspring that are not his own (i.e. cuckoldry). The risk for the female is that she might lose provisioning of offspring that are her own.

It is in the male's interest to be suspicious, but not to be paranoid (i.e. over-suspicious). If the male is paranoid rather than merely suspicious, he will fail to provide for offspring that are his own. Similarly, if the female takes promiscuity too far, she will lose provisioning for offspring that are her own. So there is a sort of "arms race" between the sexes, in which it is in the interest of each sex to exceed the other's "firepower", but only by a marginal amount. The result of any such arms race is near-parity. About as often as a female actually cheats, a male actually abandons his duties of provision.

As far as the biological benefits and risks of marriage-type arrangements are concerned, there is a remarkable symmetry between male and female.

Nature is cruel, and natural selection is indifferent to the pain it causes. And yet, love exists between males and females of species with high male parental investment. It is a warming thought that for all our obvious differences, men and women are roughly equally sincere lovers, and equally committed spouses.